This invention relates to a process for the production of a bilaterally corrugated or ribbed plastic sheet, preferably of soft PVC, which is produced in one operating step.
Such plastic sheets are utilized as seals against pressure water, seepage water, and surface water, in particular, to cover and seal inclined surfaces, for example, canal banks, dikes, or embankments (earth dams). Furthermore, it is possible to line a river bed with these plastic sheets, in order to prevent the water from seeping into the ground and, thus, the drying out of the river bed.
It is conventional to produce seals against pressure water, seepage water, and surface water by the use of bitumen or with plastic sheets which are loosely laid. However, a bituminous sheet exhibits the disadvantage that it has a permeability to water and water vapor, even though minimal, and swells under the effect of water. Furthermore, since bitumen is subject to aging, it cannot be used for durable sealing systems, as are required, for example, for ship channels.
In case of smooth plastic sheets, difficulties are often encountered when they are welded together in the field since, due to voltage fluctuations in the current supply mains and due to other disturbances, holes can be melted in the sheet, or the thus-produced welding seams are not sufficiently tight. For the purpose of sealing off inclined surfaces, such as canal banks, dikes, or embankments, plastic sheets can either not be used at all or used only with very great difficulty since, in case of rain, water collects on the sheet surface and forms a water-saturated zone, whereby the earth cover, under its own weight, slides off the sheet. When using, in place of earth, covering material such as crushed rock, larger stones, or the like, the sealing skin of the plastic sheet is endangered. Covering with prefabricated concrete slabs is expensive and requires an abutment at the foot of the bank.
Corrugated sheets are conventional which are ribbed on one side, as well as plastic sheets which are roughened by flakes made of the same material. Although this prevents the sliding off of earth masses from the corrugated or flake-covered side, the shear stresses caused by the covers on the topside of the sheet are transferred to the smooth underside of the plastic sheets. Therefore, a sliding off of the film sheets in this plane cannot be avoided if the banks are steep and the load resting on the sheet is large. The tear (or tensile) strength of the material can then be readily exceeded.
Bilaterally ribbed plastic sheets can be produced by placing unilaterally ribbed sheets with their smooth sides on top of each other and joining same. However, this procedure is very expensive, due to the plurality of working steps required. The sheets heretofore employed for this purpose exhibit rather high ribs which are arranged at large mutual spacings, so that furthermore a uniform windup of the film sheets is thereby prevented.
Also, the joining of the sheets causes difficulties, because of the high ribs at right angles to the sheet. Such places cannot be welded together with the required safe seal, since even the smallest openings must be avoided due to their capillary action.